encoding issues from IE with XML RSS Feed

I'm trying to create an RSS feed in XML 2.0. Everything works fine, except that the text data I'm pulling from MySQL doesn't appear to be encoded correctly.

I am reading the data from a website source, storing it in MySQL as plain text (no encoding being done here) then trying to reformat into XML. It works fine, unless there is a character that isn't allowed (such as the Registered trademark symbol). Then when I try to read it from IE I get the error:

An invalid character was found in text content. Error

What can I do to fix this? How can I use php to encode the text correctly? And should I do it before or after it is stored in MySQL?

Thanks for the help.